Constructed wetlands for combined sewer overflow treatment: Ecosystem services at Gorla Maggiore, Italy

Three sampling campaigns have been done in 2014-2015 in order to temporally characterize CSOs quality and quantity during representative and seasonal-specific rain events, and to estimate the ecosystem service of a CSO-CW located in Northern Italy in terms of water quality improvement. The adoption of an on-site treatment by Constructed Wetlands for the Combined Sewer Overflows is an example of decentralized approach for the optimization of the existing sewer networks and the needed reduction of their impacts on the water quality of the receiving water bodies. The CW system, that is treating the first flush of the overflows, is joined with an extended retention basin that is acting as a tertiary treatment for the CW effluents and is also storing and treating the whole second flush, slowing down by a throttle the discharge in the river and therefore playing a role in the flooding risk control. During the monitored time interval 69 events of overflow discharges have been observed and quantified; this quantification of the total volumes discharged by the specific CSO have been useful for evaluating the approach performances also in terms of mass loads in a single year, that is in the author's opinion a more conservative evaluation than comparing only the average concentrations removal rates. The obtained results have shown that on-site treatment for the first flush by CWs can obtain appropriate performances, as demonstrated by the about 87% and 93% of removal for COD and NH4+ respectively in terms of concentrations and the 68% and 94% in terms of mass loads in a year. A relevant analysis of the sampling program and the secondary services provided by this specific artificial ecosystems are discussed in the paper, together with a unique characterization of the role of the specific sources (washout, urban wastewater) to the loads that are reaching the on-site treatment unit.